Wells drilled into the ground to recover deposits of oil, gas or other desirable minerals trapped in geological formations often need to be evaluated as to the presence and particular characteristics of those deposits or as to the characteristics of the formations in which those deposits are found. After the presence of such deposits has been confirmed and a portion has been produced, additional evaluations may be performed to determine the quantity and condition of that portion of the original deposit remaining within the geological formation.
One technique for evaluating deposits and formations is to lower an evaluation tool into the well on a wireline. The purpose of some wireline tools is to measure the pressure characteristics of the formation and to retrieve a fluid sample for later analysis in a laboratory. These wireline tools have come to be known as Wireline Formation Testers or WFT's. Other methods of conveyance also exist. The term Drill Stem Testing or DST is frequently used when drill pipe or coiled tubing is used to convey the formation test tool into the well. WFT's and DST's may employ pumps to withdraw fluids from the formation or to inject fluids into the formation.
WFT's can be conveyed on a variety of different types of wireline with some standards for wireline sizes and for the number of electrical conductors having developed within the industry. Wireline sizes typically vary from 0.100 inches to 0.520 inches outer diameter, containing between 1 and 7 internal conductors. Normally two layers of external steel armour surround the conductors to provide protection and strength.
Wireline design options are constrained in several respects. The wireline must be able to fit on a spool that is capable of being mounted on a truck or on a portable skid unit. The spool itself must accommodate a sufficient length of wireline to reach the bottom of deep wells. Together, these two requirements determine a maximum possible diameter for a continuous portable wireline of any given length.
Another requirement is that the wireline must be strong enough to support its own weight, in addition to the weight of the tools to be conveyed plus an allowance for over pull in the event that the tools become subjected to frictional sticking forces. This requirement works to increase the amount of steel armour and therefore to decrease the amount of space available for the internal electrical conductors and insulating materials.
Another requirement is for high voltage ratings between the conductors and ground, as well as between the conductors themselves, if a plurality of conductors is desired. This requirement tends to increase the thickness of the insulating material that surrounds the conductors, further decreasing the amount of space available for the conducting material. Finally, the current carrying capacity of wireline increases with the diameter of the conducting material and electrical power is the product of voltage times current.
When considered together, the aforementioned design requirements all work to place an upper limit on the amount of power that can be conveyed downhole via a portable wireline. Because power downhole is necessarily in limited supply, it is prudent to make the most efficient possible use of that power which is available, particularly in those instances where the wireline tool is expected to perform mechanical work.
Conventional wirelines were first developed before the existence of WFT's and at a time when electronic technology was not in the advanced state it is today. The 7-conductor (heptacable) wireline which has become fairly standard for openhole wireline operations provided early tool designers with a plurality of signal pathways that enabled several measurements to be transmitted to the surface concurrently. Today, the need for multiple signal pathways is reduced or eliminated by the use of telemetry communications between the downhole tools and the surface equipment.
First generation WFT's did not provide for direct continuous pumping of formation fluids or of borehole fluids. Pressure drawdown measurements were made indirectly using pressurized hydraulic fluid to drive pre-test pistons moving within chambers or test-volumes. Continuous pumping capacity was not a design consideration, so that standard heptacable wireline was adequate for the purpose and hydraulic fluid pumping efficiencies were not of great concern.
While some second generation of WFT's tools do provide for direct continuous pumping of formation and of borehole fluids, the use of pressurized hydraulic fluid actuation continues. In these newer tools, the pressurized hydraulic fluid is often employed to actuate reciprocating downhole pumps, commonly referred to as mud-pumps, in addition to actuating pre-test pistons within pre-test volumes.
Hydraulic systems are known to be inherently inefficient. The overall efficiency of a hydraulic system can be calculated as the product of the individual efficiencies of all of the system components. These components necessarily include a hydraulic fluid pump with both mechanical and volumetric losses, in addition to piping, valves and other sources of frictional loss that cause heat generation in the hydraulic fluid. These hydraulic losses further diminish an already limited amount of downhole power that can be delivered to the mud-pump.
A second disadvantage of hydraulic actuation is the lack of ability to directly determine the position of the component being actuated. First generation WFT's employed pre-test designs with fixed volume chambers to address this limitation. Some second generation WFT's employing hydraulic actuation techniques require complex sensing apparatus to determine pre-test volumes or to control mud-pump through-put volumes. Frequently, this lack of ability to accurately control the volume of fluid being pumped has resulted in tool designs that continue to include pre-test volume capabilities, even though this is approach is functionally redundant in combination with a mud-pump.
A third disadvantage of hydraulically actuated mud-pumps is that the best commercially available axial piston pumps to pressurize hydraulic fluid do not provide adequate output volumes in the small diameter sizes that would be required to manufacture a high mud-pump capacity WFT of a small enough diameter to be suitable for slim boreholes. In this case it is hydraulic fluid output capacity that may become the overall limiting design constraint.
A fourth disadvantage of hydraulically actuated mud-pumps is that inherent design difficulties exist in routing power and communication links through the electric motor and hydraulic pump sub-assembly. While hollow-shafted electric motors are commercially available, hollow bore hydraulic pumps are neither commercially available nor conceptually practical to design. For hydraulically actuated mud-pump designs, this restriction necessitates the routing of power and communication links around the outside of the electric motor and hydraulic pump sub-assembly. This in turn limits the maximum outer diameter of the motor and hydraulic pump sub-assembly, reducing its potential output power, as well as greatly complicating overall assembly and maintenance tasks. While this maximum outer diameter constraint may be mitigated by routing some of the power and communication lines through the motor stator windings rather than around the outside of the motor, such approach introduces additional difficulties due to line cross-talk and transient noise from motor switching, while it further increases assembly and maintenance complexity.
Some of the other limitations of the currently available WFT's are described in the literature. W097/08424 teaches a method of well testing and intervention that combines wireline with coiled tubing to overcome the fluid injection and discharge limitations of conventional WFT's. While the method in W097/08424 might be an effective option, it is complex, costly and time consuming due to the need for large amounts of speciality surface equipment.
A second example of a limitation of existing WFT mud-pumps can be found in U.S. Pat. No. 7,395,703, which teaches the use of a complex system of controls to overcome the limitations of pre-tests that are performed in variable test volumes. U.S. Pat. No. 7,395,703 does not indicate how such pre-testing might be done as part of a continuous, rather than a discrete process.
A third example of a limitation of existing WFT mud-pumps can be found in U.S. Pat. No. 6,964,301, which teaches a method of formation sampling that uses two separate flow pathways. The first flow pathway is used to collect the sample while the second flow pathway, concentric around the first flow pathway at the inlet port, acts as a guard to limit the amount of drilling fluid filtrate entering into the first flow pathway. The intent of this arrangement is to minimize contamination of formation fluid samples. While this scheme might be partially effective, such a complex arrangement would not likely be necessary if a mud-pump of sufficient capacity were employed to ensure adequate cleanup of drilling fluid filtrate in the invaded zone prior to collecting the sample.
A recent patent which discloses formation testing while connected to a pipe string, instead of a wireline, is U.S. Pat. No. 7,594,541 (Ciglenec et al) entitled “Pump Control for Formation Testing”.
What is still needed, therefore, are simple downhole pumping techniques which make optimum use of the limited amount of power that can be supplied over wireline cables, while providing higher capacity output with pumping characteristics that are inherently useful for WFT's and that are designed in ways that make them amenable to deployment in smaller diameter formation test tools.